Higher temperature solution processes for olefin polymerization are highly desirable due to the increased throughput, decreased energy necessary for devolatization and decreased fouling that these higher temperatures afford. Although Ziegler-Natta catalyst systems can be run at high temperatures commercially, these catalysts suffer from poor efficiency and poor comonomer incorporation at elevated temperatures. In addition, polymers produced from Ziegler-Natta catalysts at elevated temperatures have broadened molecular weight distributions, which limit their suitability for use in many applications. Conventional Ziegler-Natta catalysts are typically composed of many types of catalytic species, each having different metal oxidation states and different coordination environments with ligands. Examples of such heterogeneous systems are known and include metal halides activated by an organometallic co-catalyst, such as titanium chloride supported on magnesium chloride, activated with organoaluminum and organoaluminumhalide cocatalysts. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain. The consequence of such multi-site chemistry is a product with poor control of the polymer chain architecture. Moreover, differences in the individual catalyst site produce polymers of high molecular weight at some sites and low molecular weight at others, resulting in a polymer with a broad molecular weight distribution and a heterogeneous composition. Due to this heterogeneity, mechanical and other properties of the polymers may be less than desired.
More recently, catalyst compositions based on well defined metal complexes, especially transition metal complexes such as constrained geometry catalysts (CGCs), metallocenes and post-metallocenes have been shown to give products having high comonomer incorporation and narrow molecular weight distribution. However, these catalysts often have poor high temperature stability and also suffer from poor efficiencies at elevated polymerization temperatures. Additionally, the molecular weight of the polymers formed from these catalysts often decreases dramatically with increasing temperature, especially for polymers containing significant amounts of comonomer (lower density). That is, the ability of most olefin polymerization catalysts to incorporate higher α-olefins in an ethylene/α-olefin copolymer decreases with increasing polymerization temperature, due to the fact that the reactivity ratio, r1, is dependant on polymerization temperature.
Reactivity ratios of catalysts may be obtained by known methods, for example, the technique described in “Linear Method for Determining Monomer Reactivity Ratios in Copolymerization”, M. Fineman and S. D. Ross, J. Polymer Science, 5, 259 (1950) or “Copolymerization”, F. R. Mayo and C. Walling, Chem. Rev., 46, 191 (1950). One widely used copolymerization model is based on the following equations:

where Mi refers to a monomer molecule which is arbitrarily designated as “i” where i=1, 2; and M2* refers to a growing polymer chain to which monomer i has most recently attached.
The kij values are the rate constants for the indicated reactions. For example, in ethylene/propylene copolymerization, k11 represents the rate at which an ethylene unit inserts into a growing polymer chain in which the previously inserted monomer unit was also ethylene. The reactivity ratios follow as: r1=k11/k12 and r2=k22/k21 wherein k11, k12, k22 and k21 are the rate constants for ethylene (1) or propylene (2) addition to a catalyst site where the last polymerized monomer is an ethylene (k1X) or propylene (k2X).
Certain post metallocene catalyst compositions based on Group 3-6 or Lanthanide metal complexes, preferably Group 4 metal complexes of bridged divalent aromatic ligands containing a divalent Lewis base chelating group are disclosed for use in olefin polymerizations in U.S. Pat. No. 6,827,976 and US 2004/0010103. In general, these metal complexes are usefully employed in solution polymerizations at elevated temperatures. However, their use at higher reaction temperatures in the production of ethylene/α-olefin copolymers often results in polymers having high I10/I2, due, it is believed, to high incorporation levels of long chain monomers formed in situ under the polymerization conditions employed (long chain branch formation). In many applications the presence of such long chain branching can give polymer products having advantaged properties due to increased processability and green-strength, while maintaining a narrow molecular weight distribution. However, for certain other applications, such as films, fibers and adhesives, high levels of long chain branching may not be desired. In particular, poorer tear properties in films made from such polymers, poor fiber forming and drawing properties, and diminished hot tack strength may be correlated to increased I10/I2 values. Accordingly, it would be desirable to provide post metallocene catalysts having the ability to prepare interpolymers of ethylene and one or more C3-20 α-olefins with lower I10/I2 values while retaining good high temperature operating conditions.
In addition, the solubility of this class of post metallocene metal complexes in aliphatic or cycloaliphatic hydrocarbon solvents is often lower than desired. Catalyst solubility is very important from an industrial standpoint, in order to maximize catalyst efficiency and reduce catalyst shipping volumes. The more metal complex that can be dissolved in a given volume of solvent the greater the reduction in storage and shipping costs. In addition, catalyst poisoning due to natural occurrences of impurities in the solvent, becomes significantly more problematic as concentrations are limited. A greater portion of the catalyst is sacrificed or lost due to poisoning.
Accordingly, selection of catalyst compositions capable of formation of ethylene/α-olefin copolymers at increased efficiency at elevated reaction temperatures and production of polymers of reduced or low I10/I2 is greatly desired. Use of catalysts having increased solubility in aliphatic or cycloaliphatic hydrocarbons is also greatly desired.
In US 2005/0215737 A1, a continuous, solution, olefin polymerization process is disclosed for preparing ethylene-butene and ethylene-propylene interpolymers at high ethylene conversions.
For the industrial production of high molecular weight polyolefins, especially in a continuous solution process, it is especially desirable to conduct the polymerization reaction under conditions of relatively high reactor temperature, with a high conversion of the olefin monomers to polymer in a reactor having a high solids content, all with high catalyst efficiency. This combination of process requirements severely restricts the choice of metal complex that can suitably be employed. Metal complexes that are suited for use under less demanding conditions may, in fact, be unacceptable for use under commercial processing conditions. Metal complexes that are relatively good incorporators of comonomer over wide temperature ranges with limited ability to incorporate longer chain comonomers are especially desired.
In WO 99/45041, another continuous, solution olefin polymerization process is disclosed using bridged hafnocene complexes with noncoordinating anionic cocatalysts. Although the resulting polymers contained significant amounts of comonomer, catalyst efficiencies were relatively low and polymer molecular weights, even in the absence of chain transfer agent, were less than desirable.
In WO 03/102042, a high temperature, solution olefin polymerization process is disclosed using indenoindolyl transition metal complexes to prepare polyolefins at temperatures greater than about 130° C. In one example, the copolymerization of ethylene and 1-hexene was carried out at 180° C. resulting in formation of a polymer having poor comonomer incorporation (density=0.937 g/cm3) at relatively low catalyst efficiencies.
We have now discovered that certain metal complexes may be employed in a solution polymerization process to prepare relatively high molecular weight ethylene containing interpolymers containing relatively large quantities of C3-8 α-olefin comonomer incorporated therein while retaining relatively low I10/I2 values, indicative of reduced long chain branch formation. Moreover, the solubility of such metal complexes in aliphatic or cycloaliphatic hydrocarbons (as measured by solubility at 20° C. in methylcyclohexane, hexane, or mixed hexanes) has been found to be exceptionally and unpredictably high. Accordingly, there is now provided a process for the preparation of such olefin polymer products, especially high molecular weight polyethylene interpolymers having reduced I10/I2 values, at very high catalyst efficiency.